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JAKE TAPPER, CNN ANCHOR: Anthrax, the word itself just sounds scary. How did dozens of government scientists trained to handle anthrax end up in possible danger?

I'm Jake Tapper. This is THE LEAD.

The national lead, breaking news, as many as 75 scientists working for the U.S. government now being treated for exposure to anthrax. How did such a lethal bacteria get out? Is it a danger to the public right now?

The world lead, with a militia of hard-core terrorists leaving a bloody trail across Iraq, President Obama today announces he is ready to send troops back to the nation he vowed to get us out of. Is he breaking a promise of no boots on the ground?

The politics lead, he's a former lottery winner and deli owner from a Democratic family and he will slice your Swiss cheese paper-thin if you ask him. Today, in a secret vote, Kevin McCarthy became the next leader of the Republican Party in the House of Representatives.

ANNOUNCER: This is CNN breaking news.

TAPPER: Good afternoon, everyone. Welcome to THE LEAD.

We're going to begin with some breaking news in the national lead. Just minutes ago, we learned as many as 75 scientists working in U.S. government laboratories in Atlanta, Georgia, may have been exposed to the killer bacteria anthrax.

The incident happened as researchers were transferring supplies from a high-level biosecurity lab to a lower-security facility. Those scientists are now being treated to prevent infections.

Let's bring in CNN senior medical correspondent Elizabeth Cohen.

Elizabeth, good to see you.

Do we know any more about these 75 exposed?

ELIZABETH COHEN, CNN SENIOR MEDICAL CORRESPONDENT: Jake, I was just talking to a federal official and I got sort of a feeling for what happened here.

First of all, it's really important to say none of these 75 have any symptoms of being exposed to anthrax. So there's no -- in that way, there's no worries. No one appears to have gotten sick with anthrax. But they are careful and that's why they have put these 75 folks on prophylactic antibiotics, because no harm from the antibiotics most likely and it could prevent something terrible from happening.

I got a bit of a description of what seems to have happened. So, there was a lab at the CDC that was handling live anthrax and trying to deactivate it, and they thought they had deactivated it, and they sent it out to other labs, but those other labs said, hey, wait a second, this stuff is supposed to be dead and it's alive.

So, obviously, something went wrong with the deactivation process. And another thing appears to have gone wrong. When you deactivate anthrax, you're supposed to wait 48 hours and make sure that it really is dead. And, apparently, that didn't happen in this lab -- Jake.

TAPPER: Elizabeth, when do we think this happened?

COHEN: This appears to have happened recently. I mean, they're talking about some time between June 6 and June 13. I'm looking at the CDC statement. There may have been some procedures going on in these labs that aerosolized some of those spores. So, this is really quite recent.

TAPPER: And how can they control the exposure? What are they doing now with anybody who may have come in contact with the anthrax?

COHEN: Right.

If they're concerned that someone came in contact with that anthrax, they're putting them on antibiotics. And those antibiotics are meant to prevent someone from getting sick. And, again, I think it's important to say, no one is sick at this point.

So, you know, that's certainly hopeful. If you get the antibiotics -- if anyone did get exposed -- and that's an if -- and you put them on antibiotics, that really should help.

TAPPER: Elizabeth Cohen, thank you so much.

Joining me now on the phone is Leonard Cole. He's a bioterrorism expert and author of "Anthrax Letters."

Thanks for joining us.

Have we seen a mass exposure like this before?

LEONARD COLE, BIOTERRORISM EXPERT: Well, of course, in the year 2001, immediately after 9/11, several anthrax letters, that is, letters and threats containing anthrax spores, were sent, and as a result, several people died. Five people died; 20 other -- 22 altogether were confirmed as being infected, but 30,000 people were at risk of exposure and had to take prophylactic antibiotics.

TAPPER: And what are the symptoms to anthrax exposure? How would you know? COLE: Well, in the first place, assuming that this is a highly lethal

strain that we're talking about now, and hopefully that it is responsive to antibiotics, because one could concoct a strain that would be antibiotic-resistant.

We're assuming that since this was at the CDC, they know the nature of the strain and they know that it could be susceptible to antibiotic treatment, so that's all good news. One -- in worst-case scenarios, literally within a day or two of exposure, if you have inhaled spores and if they are very lethal, one begins to get, as they say, the standard flu systems, high fever, malaise.

You get lazy. You feel sick. You get headaches. You get bone aches and then, after a day or two, in the worst case, if you don't get treatment, it could be lethal for you and beyond treatment.

TAPPER: Leonard Cole, thank you so much. And we're going to stay on top of this story.

But let's move onto the world lead right now; 4,500 American troops gave their lives in the fight for Iraq. Today, two years after the last U.S. combat troops came home, with his hand forced by a terrorist organization, President Obama said that he's ready to send up to 300 military advisers, he called them, back to Iraq to help secure Baghdad and possibly keep the whole country from unraveling.

But, at the same time, President Obama stressed that U.S. troops are not going to be fighting in Iraq again.

(BEGIN VIDEO CLIP)

BARACK OBAMA, PRESIDENT OF THE UNITED STATES: American forces will not be returning to combat in Iraq. But we will help Iraqis as they take the fight to terrorists who threaten the Iraqi people, the region, and American interests, as well.

(END VIDEO CLIP)

TAPPER: Even though these special forces are labeled advisers, instead of combat troops, one CNN military analyst says this is boots on the ground, that the parsing and loopholes all go away when someone is shooting at you.

CNN's Barbara Starr is standing by with the latest live from the Pentagon -- Barbara.

BARBARA STARR, CNN PENTAGON CORRESPONDENT: Jake, the question of course, is what exactly will these troops be doing and how will they be kept safe?

(BEGIN VIDEOTAPE)

STARR (voice-over): Iraqi forces clashing with ISIS fighters in the northern city of Kirkuk, as the militants continue to make their march towards Baghdad, President Obama finally weighing in. OBAMA: We're prepared to send a small number of additional American

military advisers, up to 300, to assess how we can best train, advise and support Iraqi security forces going forward.

STARR: With ISIS now less than 40 miles from Baghdad, the president also said the troops will keep an eye on the perimeter around the capital to see where ISIS is going next.

The U.S. troops, Army Berets, Rangers and Navy SEALs have fought in combat in the region for years. This time, the job is different. They urgently will work to get Iraqi troops and commanders back in the fight and collect intelligence about the militants' next moves in case airstrikes become necessary.

Already, fighter jets from the carrier George H.W. Bush are flying over Baghdad conducting surveillance. The ground troops will operate at several Iraqi military headquarters around the country, not on the front lines, but they may well be in danger.

Violence has even spread to Iraq's largest oil installation here in Baiji. So, how can U.S. troops be kept safe? Dozens of helicopters and aircraft now based on ships in the Persian Gulf and ashore will be ready to move in quickly if U.S. forces do come under attack, a senior defense official tells CNN.

But the president outright warning all of this can only work if the Iraqi government and Prime Minister Nouri al-Maliki move to a more inclusive government. President Obama stopped short of calling for Maliki to step down.

OBAMA: It's not our job to chose Iraq's leaders. Part of what our patriots fought for during many years in Iraq was the right and the opportunity for Iraqis to determine their own destiny and chose their own leaders.

(END VIDEOTAPE)

STARR: The U.S. troops will start arriving in the coming days, but really first a very small group, several dozen.

So why? Originally, we heard 100 troops. Now they're talking 300 troops. What's the difference? Well, what we're learning is that actually they built in an automatic plus-up. They may not get all the way to 300. They may not need them all. But by plussing up, what they don't have to do is come back in a few days or a few weeks and announce that even more troops are going to Iraq -- Jake.

TAPPER: Barbara Starr at the Pentagon, thank you so much.

Coming up on THE LEAD: But can 300 American troops there to advise, not fight, we're told, can they help hold Baghdad and save Iraq? A top general will weigh in.

And in politics, factions of the GOP facing off as the party tries to fill one of the most powerful seats in Congress. Can a conservative congressman from the most liberal state in the union bring everyone together?

(COMMERCIAL BREAK)

TAPPER: Welcome back to THE LEAD.

You're hearing from John Boehner, who is announcing the new House Republican leadership -- leadership team. Kevin McCarthy from California is the new House majority leader, Steve Scalise from Louisiana the new majority whip.

And here is Kevin McCarthy.

(JOINED IN PROGRESS)

REP. KEVIN MCCARTHY (R-CA), HOUSE MAJORITY LEADER: First and foremost, I want to thank my constituents and my colleagues for the trust that they instilled in me.

America is struggling. We're struggling with a stagnant economy, a failed health care law, and so many are living paycheck to paycheck. They're looking for individuals that put people before politics.

I make one promise. I will work every single day to make sure this conference has the courage to lead with the wisdom to listen. And we will turn this country around.

REP. JOHN BOEHNER (R-OH), SPEAKER OF THE HOUSE: Steve, you're right up.

REP. STEVE SCALISE (R-LA), HOUSE MAJORITY WHIP: Thank you.

I too want to thank my colleagues and my constituents for the honor they have entrusted me to be the next majority whip. I'm looking forward to bringing a fresh new voice to our leadership table and joining with this team to help confront the challenges that people all across this country are facing.

We have got solid conservative solutions that are going to solve the problems facing our country. We have reached out to the president to join us in solving those problems. But we're going to continue to move forward in the House as a united team, building a stronger team to address those problems and continue to work to get our country back on track and our economy moving again.

REP. CATHY MCMORRIS RODGERS (R), WASHINGTON: This has been a big day for the republicans. And I'm proud to stand with our new leadership team for the 113th Congress. Congratulations to our new Majority Leader Kevin McCarthy and new Majority Whip Steve Scalise.

More than anything, we are united. We are united in our efforts to move America forward, make America strong, to make sure that moms and dads have more opportunities to provide, bring home higher paychecks for their children and provide for their family. We are united in getting Americans back to work. We are united in getting the job done here on Capitol Hill. We want to be putting forward solutions rather than continuing to see

inaction out of the Senate. And we're going to continue to work on behalf of the people that have sent us here to make them proud and to give them more opportunities.

REP. JOHN BOEHNER (R-OH), SPEAKER OF THE HOUSE: Take a couple of questions.

REPORTER: Direct my question to Mr. Scalise. Victory was decisive. Clean, not a second ballot. A lot of people think this had to go to a second ballot here. A lot of people say the test for a whip would be able to be (INAUDIBLE). What happened? How did you have this number down that you were able to get into leadership on the first ballot?

SCALISE: We build a strong team that was representative of our entire conference. I'm proud to be the chairman of the Republican Study Committee which is 176 members of our conference. And I've worked very hard over that year and a half that I've been chairman to build consensus, to move conservative solutions forward in a way that unites our conference and solves problems facing our country.

The results that I've had over that year and a half have had broad appeal throughout our conference including non-RSC members have recognized that actually it helped strengthen our team to address the problems facing America. So, we built a really strong team, a team that's very representative of our entire conference, which shows that our conference wants to move forward even stronger so we can do a better job of addressing those problems facing the country and now have the White House and Senate start working with us to join in, in addressing those problems, as well.

REPORTER: What about those who say this is a win for the Tea Party?

SCALISE: I mean, this -- this is a win for America because we're going to be a more united team moving forward. You look at a Senate that's dysfunctional. There are so many bills we passed to get our economy moving again, to solve real problems facing hard work and taxpayers that the Senate doesn't want to act on.

The president doesn't even wants to engage, he wants to sit in the Oval Office with a pen acting like there's no legislative branch. People want to see people in Washington working together to solve real problems. I think what this message sends is that the House is very united to address those and now we're waiting. Let the Senate and president follow suit.

REPORTER: Mr. Speaker, I have a question for Mr. McCarthy.

Sir, there are probably a lot of grassroots Republicans out there, the kind that voted for Dave Brat against Eric Cantor saying, are you guys kidding me? We got him out because we wanted a more conservative forgive leadership and they elected a guy from one of the bluest states in the Union, California.

REP. KEVIN MCCARTHY (R-CA), MAJORITY LEADER: They elected a guy who is a grandson of a cattle rancher, a son of a firefighter. Only in America do you get that opportunity.

They elected a guy that's only grown up through the grassroots. They elected a guy that spent his time going around recruiting many of these individuals to get the majority.

Look, I've always had to struggle for whatever we wanted to overcome. I think that's the greatest part about America, that they've always given you the privilege and the opportunity. That's what this party brings, as well. I think given an opportunity, people will be very impressed about what we're going to do and where we're going to go.

REPORTER: One more question.

BOEHNER: Thank you. Thank you, thank you.

JAKE TAPPER, CNN ANCHOR: You were just listening to the House Republican leadership announcing their new slate of leaders. Obviously, House Speaker John Boehner still there. The former Majority Leader Eric Cantor defeated just a week or so ago in a surprise primary election. And Eric Cantor stepping down and the new House Majority Leader Kevin McCarthy of California arguably less conservative than Eric Cantor.

Dana Bash, our chief congressional correspondent is on the phone -- oh, in person, there you are right now with us.

Dana, so McCarthy something of a surprise to Tea Party members probably, but Steve Scalise, very conservative from Louisiana.

DANA BASH, CNN CHIEF CONGRESSIONAL CORRESPONDENT: Very conservative. He was the head of the Republican Study Group which to many viewers probably won't mean much, but to core conservatives means a lot because it is the group of Republicans in the house caucus who are the most conservative. Until he just got this job as the new number three, he was that guy. He also is now the one red state Republican at the leadership table.

So his actual role which is House majority whip is critically important to any majority in the House because that's the person that counts the votes. And the way the House is run, the majority does not want to bring a piece of legislation to the floor unless they're pretty sure, almost entirely sure, that they're going to get the magic 218 votes that's needed to pass.

So that's why that whip job so important on a tactical legislative level, but this particular election of Steve Scalise is important also as a Republican unity message, as well. Making clear to conservatives in the House, red state Republicans, Republicans who wanted somebody they thought was more one of their own at the leadership table, that there is more inclusive GOP leadership.

TAPPER: Bob Costa from "The Washington Post" also joins us right now.

Bob, an interesting issue when it comes to this House Republican leadership slate is the issue of immigration reform. Obviously, it's been contentious. Speaker Boehner wanting immigration reform. Eric Cantor his mind having been somewhat opened to it at one point. He got hammered for that in his primary and lost. Kevin McCarthy, the new House majority leader, he supports giving some sort of legal status to undocumented workers.

Steve Scalise, the new whip, the new member of the leadership team, not only against immigration reform, not only speaking against what he calls amnesty, he's somebody that opposes giving automatic citizenship to people born in the United States. How on earth is this group going to be able to work together on immigration reform?

BOB COSTA, THE WASHINGTON POST: It's going to very difficult for advocates of immigration reform. But I think the ascension of McCarthy to the majority leader position is a sign of hope for those who would like to see some kind of comprehensive reform.

McCarthy, as you said, does approve giving legal status to undocumented workers. So does Boehner. I think these two could collaborate on some type of project.

TAPPER: Are they going to be able to be ask Steve Scalise, the red state Republican everyone Louisiana who opposes even having guest workers in this country, who opposes automatic citizenship for people born in the country, are they going to be able to get him to whip as his job requires for something that he opposes?

COSTA: We'll see. When we look what Steve Scalise means inside the U.S. House, he has a lot of political capital, but he doesn't carry every single conservative with him. He's not a conservative heavyweight in the same way Paul Ryan is. In fact, Paul Ryan, I think, could be the greatest ally to Boehner and McCarthy, as they pursue immigration reform. He wants to get something done.

Scalise, he had some conservative opposition in this whip race. If he's going to be the one who stops it, he's going to have some others around him who break with Boehner and McCarthy.

TAPPER: And, Dana Bash, our chief congressional correspondent, just to remind people exactly what these jobs are, the House Speaker John Boehner, he runs the entire House, Democrats and Republicans. What does the majority leader do? What does the whip do?

BASH: OK. So, the House majority leader is the one who's in charge of the calendar. Decides when the House will be in session, when the House will not be in session. And more importantly, decides what legislation is going to go on the floor.

Now, the House speaker is a constitutional role. There's second in line to the president. So that is supposed to be the speaker for the whole House the way is in modern times, probably since the beginning of time. It's more of a political role. So, the majority leader, would with the house speaker on those issues.

And, then the number three, the whip as I mentioned before is in charge of counting the votes which is why your question to Bob was so important. You have a conservative in that role who might not agree with some of the legislation that the leadership wants to put on the floor in the next couple of years. So, is he going to be able to do that job of convincing people to vote for something at the personally might not vote for? That's one of the questions of leadership in general.

But one other point I want to make, Jake, is that -- on the question of immigration which I think is fascinating. Kevin McCarthy, the new House majority leader will who will be in charge of putting legislation on the floor, he is different from a lot of his colleagues in that because of the partisan nature and the gerrymandering of the congressional districts, most Republicans do not represent districts with big Latino populations. Kevin McCarthy is different. His district in California has one of the highest among Republicans, one of the highest populations of Latino voters.

So, for him, it is personally it's not politically dire or a political suicide to push this kind of thing forward.

So, you know, for Eric Cantor, you know, maybe one of the reasons why he got beaten by a fellow Republican. Someone like Kevin McCarthy is safe on that issue. That might help him push forward this legislation which they all say they want do in the -- at the end of the Obama term.

TAPPER: Bob Costa with "The Washington Post," just one final thought, Steve Scalise, the new House majority whip from Louisiana, this was a competitive race, and people thought it was going to go to the second ballot. I even heard some comments about that it might go to the third. It didn't at the end of the day. Why not?

COSTA: Scalise was able to make a regional argument. A lot of conservatives from the Deep South felt they needed a representative within the upper echelon of House leadership. He really rode that case all the way to the third most powerful position in the House. He was able to edge out Peter Roskam from Illinois and Marlin Stutzman, another Midwesterner from Indiana. That was really the core of his support, the Southern conservatives who are frustrated with the current leadership.

TAPPER: Dana Bash, Robert Costa, thank you so much. We appreciate it.

Coming up on THE LEAD, the president putting 300 military advisors on the ground in Iraq. What exactly can they do? We'll ask a general and former CentCom commander, coming up next.